The invention generally relates to a technique to regulate an efficiency of a fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. The membrane is sandwiched between an anode catalyst layer on one side, and a cathode catalyst layer on the other side. This arrangement is commonly referred to as a membrane electrode assembly (MEA). At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:H2→2H++2e− at the anode of the cell, andO2+4H++4e−→2H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair are often assembled together in an arrangement sometimes called a membrane electrode unit (MEU).
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas, propane methanol, as examples) into the fuel for the fuel cell stack. For a given output power of the fuel cell stack, the fuel and oxidant flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel and air flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel and air flow and controls the fuel processor accordingly.
Due to nonideal characteristics of the stack, it may be difficult to precisely predict the rate of fuel and air flow needed for a given output power. Therefore, the controller may build in a sufficient margin of error by causing the fuel processor to provide more fuel and/or air than is necessary to ensure that the cells of the stack receive enough fuel and thus, are not “starved” for fuel or air. However, such a control technique may be quite inefficient, as the fuel cell stack typically does not consume all of the incoming fuel, leaving unconsumed fuel that may burned off by an oxidizer of the fuel cell system.
Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are recited above.